Recently, Professor Wu's research group published a new research of “Synergistic Incorporation of NaF and CsF PDT for High Efficiency Kesterite Solar Cells: Unveiling of Grain Interior and Grain Boundary Effects” in the Journal of Materials Chemistry A (J. Mater. Chem. A 2020, DOI: 10.1039/D0TA08224D).

The introduction of alkali metals (AMs) post-deposition treatments (PDT) have led to the consecutive breakthrough of world record efficiencies for Cu(In,Ga)Se₂ photovoltaic. While for its derivative Cu₂SnZn(S,Se)₄ (CZTSSe) solar cells, the inherited verdict seems partially unjustified due to the difference in defect chemical environment and phase structure, and the most effective incorporation means of AMs is still a mystery. In this work, we firstly build up a dual AMs synergistic incorporation project combined light and heavy AMs by post-depositing CsF after NaF-PDT procedure to improve the efficiency of CZTSSe solar cells. The relevant microscopic-electrical advantages with regard to different role of NaF and CsF PDT are discussed in more detail. It is turned out that Na atoms are randomly dispersed both in grain interior (GI) and grain boundaries (GBs), significant increasing the acceptor concentration and passivating the deep level defects to enhance V_oc. While Cs atoms are mainly segregated at GBs, primarily increasing GBs potential and more inclined to improve FF. The prominent electric benefits of Cs at GBs and Na in GI finally increase the device efficiency from 10.12% to 12.16%. Our findings solve the difficulty in obtaining available V_oc and FF for future high efficiency research and should serve as a basis for depth understanding of AMs-PDT in kesterite photovoltaics.

The first author of this paper is Xiaohuan Chang and Dr. Junjie Fu of Key Laboratory of Special Functional Materials, and the corresponding authors are Professor Sixin Wu and Associate Professor Dongxing Kou. This work is supported by the National Natural Science Foundation of China, the Science and Technology Department of Henan Province, and the Education Department of Henan Province.